Implanted medical devices are sometimes subjected to impact from externally applied forces, such as those associated with the falling of a patient or a blow to the patient's body in the region of the implanted medical device. While most externally applied forces do not result in damage to or failure of an implanted medical device, implanted medical devices including an antenna coil assembly such as that of a cochlear implant, are known to have been damaged by impact forces to the head of the patient where such implants are located. Such failed devices are surgically removed and replaced with a functioning device, and examined to determine the failure mechanism. Applicants' examination of such removed devices has revealed that the failure mechanism was commonly a tensile breakage of wire in the implanted antenna coil assembly. Further analysis by the applicants has revealed that the wire breakage was the result of impact forces, especially forces non-orthogonal to the plane of the implanted antenna coil assembly. Such non-orthogonal impact forces included lateral components reflected as tensile forces in the plane of the coil assembly sufficient to fracture the wire. Such impact forces may result from a fall or a glancing blow where the impact is made non-orthogonal to (i.e., other than normal to) the head. Because these devices are implanted subcutaneously, the impact resistance of the device is at least in part dependant upon the thickness of the skin flap that covers the device. While thicker skin flaps of adults afford some impact protection, children often have skin flaps of only 1 to 3 mm, and therefore are at more risk of breaking the coil wire.
U.S. Pat. No. 6,542,777, issued Apr. 1, 2003, which is assigned to the assignee of the present invention and incorporated herein by reference, describes and illustrates an invention directed to the use of a spiral shield that improves the electrical behavior of an implantable secondary coil of an implantable device. To maintain the spacing of the turns of the secondary coil during the manufacturing process, a polymeric spacer is placed between adjacent turns. The coil, spacer, and shield then are encapsulated in a biocompatible polymer, which is preferably the same polymer used for the spacer (column 2, lines 42-45 and column 3, lines 23-27), or a material with similar physical characteristics (column 5, lines 40-43), with examples given as SILASTIC® or SILBIONE® LSR 70 (column 6, lines 8-12). At column 3, lines 16-23, the '777 patent states that the combination of the biocompatible polymer encapsulation, multi-strand wire, and spiral shield results in a thin, flexible, secondary coil assembly that resists damage from impacts. While that may be true, there is no teaching or suggestion in the '777 patent of any means for absorbing non-orthogonal impact forces. Because the spacers are made from the same material or a material with similar physical characteristics as the encapsulant, such as 70 durometer silicone rubber, the overall structure is uniform, and the final molding is the same whether manufactured with or without spacers. Further, applicants have found that such a silicone encapsulated secondary coil assembly may be subject to non-orthogonal impact forces having lateral components reflected as tensile forces in the plane of the coil assembly sufficient to fracture the coil wires within the antenna coil assembly.
Accordingly, there is an immediate and continuing need for an improved impact resistant implantable antenna coil assembly for medical devices that will absorb non-orthogonal impact forces and particularly the lateral components thereof that would otherwise be reflected as tensile forces in the plane of the coil capable of fracturing the wire. The present invention satisfies that need.